In what follows, in a purely illustrative and non-limiting manner, reference will be made generally to the medical field.
Document discloses EP-345 133 a surgical implant destined to be assembled to a bone using a screw. The surgical implant described in this document includes a passage through which passes the screw, and a countersink with external threading of which the diameter is greater than that of the head of the screw, so as to define a root constituting a support surface for said head.
A check screw is received and screwed into this countersink, by exercising on the head of the fixing element an axial compression effort which applies it against the root of the countersink.
Such a fastening principle, which consists in solidly fastening and blocking the head of the screw relative to the implant by the check screw, makes it possible to benefit from a sealing effect realized by the anchoring of the screw in the bone on one hand, and the fastening of the implant on the screw on the other hand.
Thus, contrary to more traditional arrangements, the positive connection of the bone and implant described in document EP-345 133 does not depend on a compression force that applies them together, nor consequently on the forces of friction at the level of the implant/bone interface.
Consequently, even when the bone matter is degraded at the level of the implant/bone interface, the implant described in document EP-345 133 remains mounted in a stable manner, since the bone/implant contact is not imperative in order to ensure that the mounting holds.
This character of stability favors bone reconstruction, and makes it possible to minimize surgical and postoperative difficulties, which contributes to restoration of the patient in the best conditions.
The device described in document EP-345 133 has however the inconvenience of not being able to freely fix the orientation of the screw relative to the implant.
The device described in this document only authorizes in fact a single angular positioning of the screw relative to the implant.
However, it is particularly interesting to have a system that can be oriented at will, in particular in traumatology, for example to approach an isolated bone fragment.
Furthermore, in the particular case where the implant is an osteosynthesis plate, it is shown to be generally indicated to vary the angles of the security screw relative to the plate, so as to benefit from anchoring according to axes that cross amongst themselves, which limits the effect of pulling of the plate when the latter is subjected to mechanical efforts of the type normal traction, bending or other.
In this case, it is shown to be interesting to be able to select, at the time of placing, the screw angulations, so as to adapt them to the particular anatomical constraints of the patient operated on.
Document FR-2 790 198 discloses an implant system making it possible to freely determine the screw angulation in relation to the implant.
The system described in this document includes an implant equipped with an orifice within which is positioned a radially expanding ring wherein the security screw is screwed.
Said screw has a radial enlargement near its head, such that at the end of screwing, this enlargement causes the ring to expand radially and the blocking of the latter in position against the walls of the orifice.
Such a system does not make it possible however to benefit from a sealing effect equivalent to that provided by the arrangement described in document EP-345 133 previously mentioned.
Moreover, the system described in document FR-2 790 198 requires the use of an instrument to tighten the screw as well as an instrument for blocking the ring during the tightening of the screw. Consequently, the two hands of the surgeon are solicited simultaneously, which complicates the operating task.
Furthermore, the arrangement proposed by document FR-2 790 198 exposes the ring to excessive expulsions from the orifice, since the ring is retained within the orifice only and exclusively par the forces of friction generated by the radial pressure exerted by the radial enlargement of the screw.
Finally, in the system described by document FR-2 790 198, blocking the screw in position relative to the implant occurs at the same time as the tightening of the screw in the bone, according to a single tightening gesture. Other than the fact that such a confusion of tasks does not make it possible to realize accurately, in terms notably of intensity of effort, the tightening in the bone as well as the blocking in angular position, certain mountings are revealed in practice to be extremely delicate to realize. This is the case for example with piled mountings, where the implant, after mounting, is located at a considerable distance from the bone.